Most of us only think about circuit breakers when one trips because we plugged in too many appliances and then tried to also run the vacuum cleaner. A quick trip to the basement to flip a switch allows us to quickly go on with our lives. What you may not have realized, though, is how complex these seemingly simple devices really are. Circuit breakers not only protect people from dangers caused by electrical system faults within their homes, but also protect the people, wiring and machines connected to or using nearly every electrical circuit or system in the world today.
One of the most critical functions of a circuit breaker is to interrupt a short circuit. This can happen, for example, if a child sticks something made of metal into an electric outlet or a wrench falls onto conductors in an industrial setting. An uninterrupted short circuit can injure people, start fires, and destroy equipment. GE’s circuit breakers use advanced technology to produce some of the best short circuit interruption capability on the market.
As circuit breaker technology improves, breakers are able to protect circuits with larger and larger amounts of available fault current. Available fault current is the highest amount of current that can flow through a circuit during a fault or short circuit, and varies based on the characteristics of the local power grid. As the fault current rating of the breaker increases, so does the power that must be dissipated in the breaker during a short circuit event. It is this increase in power level that has been the catalyst behind the major innovations in circuit breaker technology.
[The video above shows the generation and movement of the electrical arc created when a circuit breaker opens under low voltage conditions.]
The breaker that GE’s Industrial Solutions business is developing today for the industrial market will dissipate 2.7MW of power. This is equivalent to 3,620 horsepower, or what you’d get in seven Porsche 911 Turbos. To accomplish this we have to separate the contacts quickly; we hit 30 mph average velocity, starting from zero and reaching full open in less than 2 milliseconds. An arc is created when the contacts part. This arc is at least 6,700°C and can reach temperatures as high as 19,500°C – more than three times the temperature on the surface of the Sun. This also creates pressures inside the breaker of 250-300 psi or 17-20 atmospheres. The breaker must be designed to withstand these pressures while limiting the exhaust temperature and controlling the molten metal particulates created by the high-temperature arc. While all of this is happening, the force between the current carrying parts in adjacent phases can reach 5 tons due to the electromagnetic forces. All of this happens in less than a second in a volume much smaller than the inside of your microwave oven!
[The video above shows a short circuit test of an existing GE product performing at the higher 200kA/480V level that researchers are now working to accommodate.]
Breaker design is an interdisciplinary field. At GE Global Research, a diverse team of electrical and mechanical engineers, physicists, manufacturing experts, materials scientists, and metallurgists are involved in researching critical components that must withstand incredibly extreme conditions.
In order to deal with the complex nature and demanding requirements of circuit breaker design, our research team is working on new modeling and simulation tools that will allow us to simulate the electromagnetic, mechanical, and fluid dynamic aspects of breaker behavior and study the impact of each aspect on the other in a multi-physics simulation environment. A joint program with GE Industrial Solutions, GE Global Research, and the University of Connecticut will allow us to design better breakers faster and at a lower cost to our customers.
There are many significant challenges that our team must overcome before a multi-physics model can be created that truly characterizes the full behavior of the circuit breaker. One of the key problems to be solved is an accurate prediction of the arc root movement and the associated properties of the arc plasma inside a circuit breaker case. This requires an accurate model of the impact of the electromagnetic forces on the arc and an understanding of the impact of those forces on the shape and position of the arc, which then impact the arc’s electrical properties. The arc’s electrical properties are also impacted by the mechanical properties of the system, the temperature and pressure and the molten metal and vaporized polymers.
[The video above shows a short circuit test of a circuit breaker in a metal box enclosure. Sixty-five thousand amperes is applied through the power cables that enter through the hole in the box and connect to the circuit breaker. You can see the extremely high pressure hot gas and particles exiting the box through this hole and through the flexed door on the right.]
Our team is using advanced coupled finite element and finite volume software, techniques in plasma physics, and new methods for parsing the problem to finally close the loop on a multi-physics model for circuit breaker design. This will allow us to optimize design in ways we’ve not imagined in the past. For example, using these modeling techniques we will be able to design a breaker optimized for our industrial customers’ needs in significantly less time than it would have previously taken us. This will allow our engineering teams to spend a lot more time creating the breakthroughs that will bring us the next generation of circuit breakers.